1. Field of the Invention
This invention relates to a variable equalizer which, when the frequency characteristic of a transmission line has changed in an electrical communication system, is capable of varying the frequency characteristic in order to compensate for the characteristic change.
2. Description of the Prior Art
In a wire communication system employing a coaxial cable, a pair cable or the like, the transmission characteristic of the cable varies depending in dependence on the length of the transmission distance, fluctuations in the ambient temperature, etc. It is therefore necessary to make compensation for this variation with some networks equipped in repeaters.
The response of the transmission line vs. frequency, when represented in decibel, changes in proportional to the reference characteristic which is inherent to the transmission line.
For compensating for the characteristic changes or gain changes, the Bode type variable equalizer (refer to, for example, H. W. Bode, "Variable Equalizer," Bell Syst. Tech. J., Vol. 17, No. 2, pp. 229 - 244, 1938) is often used. The transfer characteristic e.sup..psi. of this variable equalizer is represented as follows, by letting x denote a variable coefficient and e.sup..phi. (where .phi. is a function of the length and the frequency) denote variable reference characteristic: ##EQU1## Expanding .psi. in terms of .phi. yields: ##EQU2## WHERE E(X).multidot.F(.phi.) DENOTES AN ERROR TERM. The error increases exponentially as .phi. (the variable range) becomes larger. It has therefore been impossible to use a too large value for .phi. (a very wide variable range).
In order to solve this problem, one of the inventors of this invention has previously proposed a variable equalizer whose characteristic or variable transfer function is described as ##EQU3## The realization of this variable function, however, was too complicated for practical use.